Conventionally, there is proposed a technology of improving detection accuracy of a laser radar device that projects a laser beam to a forward direction of a vehicle to simultaneously receive reflected light from different directions in a horizontal direction using a plurality of light receiving elements.
For example, according to one proposal, a plurality of light receiving elements are selected with any combination, and light reception signals output from the selected light receiving elements are added and output to increase light reception sensitivity. According to one proposal, in the case where vehicle speed exceeds a predetermined threshold, a range of the light receiving element to be selected is narrowed to narrow a view angle, the plurality of light receiving elements are selected while shifted one by one, and the light reception signals are added, whereby the light reception sensitivity is increased without degrading resolution in the horizontal direction. According to one proposal, in the case of a determination that a reflector does not exist in front of the vehicle, the plurality of light receiving elements are shifted and selected so as not to be overlapped with each other, and the light reception signals are added, whereby the light reception sensitivity is increased while the resolution in the horizontal direction is decreased, and high-speed scanning is performed (for example, see Japanese Unexamined Patent Publication No. 7-191148).
Conventionally, in a vehicle laser radar device that horizontally scans a pulsed laser beam, there is also proposed a technology of restraining intensity of the laser beam in order to secure safety of a pedestrian or a passenger in the vehicle in front of an own vehicle.
For example, according to one proposal, the intensity of the laser beam can be controlled in each angle at which the laser beam is output, the intensity of the next laser beam is decreased in a direction in which the intensity of the light reception signal exceeds an upper limit, and the intensity of the next laser beam is increased in a direction in which the intensity of the light reception signal is less than or equal to the upper limit. According to one proposal, a detection distance necessary for each angle of steering wheel is decided based on an angle of steering wheel and vehicle speed, and the intensity of the laser beam to each angle is controlled. According to one proposal, in the case where the vehicle is detected in front of the own vehicle, the intensity of the laser beam is decreased in the direction of the detected vehicle. According to one proposal, in the case where a short-range obstacle is detected, spread of the laser beam is increased in the direction of the detected obstacle, or an emission cycle is lengthened (for example, see Japanese Unexamined Patent Publication No. 7-167958).
According to one proposal, for example, preliminary emission and main emission are alternately repeated during one scanning period, the main emission is performed with normal intensity higher than that of the preliminary emission in the case where the obstacle is not detected by the preliminary emission with the laser beam of the low intensity, and the main emission is performed with the intensity equal to that of the preliminary emission in the case where the obstacle is detected by the preliminary emission. According to one proposal, the main emission is performed with the normal intensity in the case where the obstacle detected by the preliminary emission is not located near the own vehicle, the main emission is performed with the intensity equal to that of the preliminary emission in the case where the obstacle is located near the own vehicle. A pulse width of the main emission is increased in the case where the obstacle detected by the preliminary emission is not located near the own vehicle, and the pulse width of the main emission is decreased in the case where the obstacle is located near the own vehicle (for example, see Japanese Unexamined Patent Publication No. 2002-181937).
According to one proposal, in a laser radar device in which the light reception signals of the laser beams of plural times are integrated to detect the object while the pulsed laser beam is horizontally scanned, scanning speed of the laser beam is decreased in an area where a reflected light amount is smaller than a predetermined value, and the scanning speed of the laser beam is increased in an area where the reflected light amount is larger than the predetermined value. Therefore, detection accuracy of the low-reflectance object is increased, and time necessary for one-time scanning can be shortened without decreasing the detection accuracy of the high-reflectance object (for example, see Japanese Unexamined Patent Publication No. 2012-63236).
In the case where the laser radar device is used in an urban area, because the vehicle runs at low speed, it is not necessary to monitor a distance. On the other hand, when sensitivity of the laser radar device is excessively increased in order to monitor the distance, because of many stationary bodies such as buildings in the urban area, the object having lower detection necessity is also detected to degrade reliability for a detection result of the laser radar device.
A method for increasing the intensity of the laser beam is conceivable as one of the methods for increasing the sensitivity of the laser radar device. However, the excessively strong laser beam is possibly harmful to a human body. Accordingly, it is impossible to immoderately increase the intensity of the laser beam. There is a problem that the number of components is increased for the change in intensity of the laser beam.
On the other hand, in the case where the vehicle runs at high speed on an expressway or a suburban highway, there is a demand for detecting the farther object, such as the vehicle in front of the own vehicle, in order to avoid collision of contact.